For example (Input –> Output):

39 --> 3 (because 3*9 = 27, 2*7 = 14, 1*4 = 4 and 4 has only one digit)
999 --> 4 (because 9*9*9 = 729, 7*2*9 = 126, 1*2*6 = 12, and finally 1*2 = 2)
4 --> 0 (because 4 is already a one-digit number)

The problem link is here.

Solution:

int persistence(int n)
{
  int x = 0;
  while (n > 9)
  {
    int m = 1;
    while (n)
    {
      m = m * (n % 10);
      n = n / 10;
    }
    n = m;
    x++;
  }
  return x;
}

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